The purpose of a catalytic converter is to convert pollutant materials in engine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc. to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square or triangular, straight-through openings or cells; catalyst coated alumina beads; or a corrugated thin metal foil honeycomb monolith having a catalyst carried on or supported by the surface, which surface is typically wash coated with one or more metal oxides, such as alumina, ceria, lanthia, or combinations thereof, and a catalyst. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst catalyzes a chemical reaction whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere. However, this conversion is not efficient whenever the exhaust gases are relatively cold. To have high conversion efficiency, the catalyst and the surface of the converter with which the exhaust gases come in contact must be at a minimum temperature above about 200 F., e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC), and 1000 F. for methane or natural gas. Otherwise the conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. For example, about 80% of automotive pollution occurs within the first two minutes of engine operation. Once the exhaust system has come to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases be heated or contact a hot catalyzed surface to effect satisfactory conversion at start-up.
To achieve rapid heating of the catalyst in a metallic monolith, it is necessary to draw power from a voltage source, or other source of electric energy, e.g., a battery or a capacitor device (such as the new Isuzu "electric power storage" device developed by Isuzu Motors Ltd.) until the desired temperature is reached. In an automotive vehicle, this source of energy is usually a 12 volt or a 24 volt battery, although a battery system supplying higher voltage may be used. As indicated, a battery, or a capacitor battery or, if an immobile source, a conventional AC or DC power supply may also be used as a source of energy. To accomplish this end, it has been found that one or more actuatable solid state switches connected in parallel, such as metal oxide semiconductor field effect transistors (MOSFETs) together with means for actuating such devices in unison (a gate driver) may conveniently be used. This system in vehicles enables drawing high power loads for a short period of time to achieve the desired temperature in from broadly upwards of 0.5 second, desirably above about 2 seconds, e.g., from 2 to 30 seconds, and preferably from 2 to 60 seconds of precrank preheating, and, if necessary depending upon the specific application, from 0 to 60 seconds of postcrank heating.
Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated Oct. 30, 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,389 to Kitzner dated Oct. 30, 1973 which discloses a central electrically heated core within a ceramic monolith. The heating core is formed of metal sheets, one corrugated, the other flat, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. However, heating by conduction requires too long (a matter of minutes) to be practical in solving the problem of atmospheric pollution at start-up.
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison and Retallick dated Dec. 8, 1987 for details of a process for the preparation of an accordion folded thin stainless steel foil honeycomb with an alumina coating and a catalyst on the surface thereof. This patent is incorporated herein by reference thereto.
Reference may also be had to International PCT publication number WO 89/10471 filed Nov. 2, 1989 which discloses electrically conductive honeycomb catalyst support units useful in automobiles. To obtain a suitable electrical resistance between 0.03 and 2.0 ohms, the honeycomb body is subdivided electrically, cross-sectionally and/or axially, by gaps and or electrically insulating intermediate layers or coatings so that at least one electrical current path having the desired resistance is obtained. Heating is controlled by a timed relay. Separate catalytic converters in the exhaust line, one or more electrically heatable, the other conventional, are disclosed. The minimum resistance disclosed is 0.03 ohms which when placed in a nominal 12 volt electrical system, as disclosed, may be expected to draw a current of no more than about 300 amps at the catalyst when the various voltage drops and resistances are considered. This power level has been found to be too low for effective rapid heating of electrically heatable units within from 0.5 to 60 seconds and, even if the monolith could be heated rapidly at this power level, the catalyst tends to cool rapidly to below "light off" temperature after the engine is started because of the initial cool exhaust from the engine. To counteract this cooling effect usually more than 2700 watts (less for smaller engines) are required after engine start-up. It should be noted that on some vehicles, this cooling to a lower temperature is not important if it occurs during a period of time when the engine exhaust emissions are low, in which case a lower power level (down to 500 watts) may suffice. Moreover, subdivision of the monolith into a plurality of discs or units for connection in series is not necessary.
Many millions of automotive vehicles are equipped with catalytic converters, but all are subject to cold-start emissions of what, in at least one state, has been determined to be an unacceptable level. Anticipatory elevation of the catalyst temperature to an optimum operating level before start-up is expected to be mandated for many automotive vehicles.
Moreover, the systems of the present invention will achieve an effective pollutant removing temperature, or "light off" temperature in about two seconds depending upon the power delivered from the source of electrical energy. This compares with over five minutes for converters with no auxiliary electrical heating means or those which depend upon heat conduction as the mode of heat transfer. This is an important distinction if one is obliged to wait for a catalytic converter to achieve "light off" or optimum operating temperature before starting the engine. Again, it is important to note that a major amount of pollution due to exhaust gases occurs in the first two minutes of engine operation. Some applications of the devices contemplated hereby use power supplied continuously, i.e., without a power modulating system.